1. Field of the Invention
This invention pertains generally to an apparatus and process for the treatment of wastewater and biological nutrient removal in activated sludge systems. The apparatus facilitates universal equipment providing substantially steady agitation while accommodating alternating process conditions (such as anaerobic, anoxic, aerobic, and oxic conditions) in a reactor.
2. Prior-Art
Municipal and industrial wastewaters contain significant quantities of phosphorus and nitrogen, and the removal of these nutrients has become an important facet of wastewater treatment. In a wastewater treatment plant, phosphorus and nitrogen can be removed by both biological and physical chemical means. Biological means of nutrient removal are generally preferred, as they result in lower waste sludge production, produce a sludge that is more amenable to land application, and have the public perception that biological processes are more “environmentally friendly” than chemical processes. Processes using biological mechanisms for phosphorus and nitrogen removal are generally referred to as biological nutrient removal, or BNR, processes.
Biological nitrogen removal in the activated sludge process takes place in two sequential reactions—nitrification and denitrification. Nitrification is the biological oxidation of ammonia to nitrate and nitrite by two specialized groups of autotrophic bacteria that takes place under aerobic conditions. Denitrification is the biological reduction of nitrate and nitrite to nitrogen gas that takes place under anoxic conditions. During the 1960s, North American research focused on the development of two- and three-stage processes for nitrogen removal, with separate stages for carbon removal, nitrification, and denitrification. Each stage had the prerequisite conditions required to sustain its biological reaction followed by a set of clarifiers. Researchers in Europe, meanwhile, developed single sludge systems in which all of these reactions take place simultaneously in a single process in which the sludge is sequentially subjected to anoxic and aerobic conditions. Recent examples of single sludge applications are SHARON (Single reactor system for High activity Ammonium Removal Over Nitrite) and ANAMMOX (ANoxic AMMonium OXidation).
Some of the technologies focused on satisfying high oxygen requirement per unit volume and therefore targeted to hold more biological activated sludge in activated sludge reactors. For example, UNOX system developed in the 1960s used high purity oxygen (HPO) as an alternative to air provided by surface aerators and upgraded existing aerobic reactors by simply altering the same infrastructure and covering aeration tanks. In the 1970s, open-tank HPO systems were developed such as the BOC VITOX system by British Oxygen Company, eliminated the confined space limitations and exhaust-gas troubles of the UNOX system and also made possible to have deeper aeration tanks in resulting footprint reduction and capital cost savings.
In the late 1980s, by incorporating an engineered plastic media to activated sludge system Moving Bed Bio-Reactor (MBBR) systems were developed and combined suspended-growth and attached-growth advantages into one system. By adding a recycle activated sludge line to the MBBR system, integration of suspended-growth and attached-growth was further enhanced and was referred to as Integrated Fixed-film Activated Sludge (IFAS). In the 1990s, the Membrane Bioreactor (MBR) was developed as a robust solid-separation mechanism, and integrated into activated sludge systems to meet more stringent suspended solids and phosphorus effluent targets.
MBR, MBBR, and IFAS systems are often called hybrid systems. Most of the hybrid systems' performances rely on aeration method they use. In most existing aeration devices, rate of hydraulic-mixing and degree of aeration are simultaneously dependent on each other as, for example, in surface aerators and air-blowing diffused aeration systems (such as those disclosed in U.S. Pat. No. 6,372,140). When more air is required, more mixing is inadvertently and unnecessarily provided.
Excessive mixing energy and agitation in activated sludge system can cause adverse effects on system performance, such as a pin-floc problem in suspended-growth systems or excessive bio-film sloughing-off in attached-growth systems, which in turn can lead to sedimentation and solids separation problems.
MBR, MBBR and IFAS systems are examples of wastewater treatment systems that are the most vulnerable to the problem of excessive agitation. All of those systems often use diffused aeration and therefore their efficiency relies on that particular aeration method's pros and cons. For example, MBR systems rely on micro-filtration taking place in an activated sludge reactor with high level of suspended solids, and therefore usually require a high degree of agitation in the aeration reactor to keep the membranes' surfaces clean and reduce their reject time. However, excessive agitation has adverse effects on the treatment performance mentioned above. Some of the MBBR and IFAS systems rely on the development of bio-film on small, lightweight, rigid plastic floatable carrier elements that fill the aeration basin and are kept agitated by means of diffused aeration. Homogeneous mixing of MBBR and IFAS plastic floating media has been a challenging issue since the mixing of floating media by means of diffused aeration is more challenging then mixing of settling solids. (Conventional activated sludge systems do not contain artificially added floating media and therefore are relatively less vulnerable to this problem.) Excessive agitation is definitely a serious problem and limiting the theoretically expected actual performance of MBR, MBBR and IFAS systems.
There are commercially available aeration systems that provide an independent aeration-rate with respect to the hydraulic-mixing-rate, such as BOC VITOX, MTSJETS and another system disclosed in patent WO/2001/002308. However, due to their potential high energy requirements (for air blowers or oxygen-generators in addition to liquid recirculation pumps), as well as their complexity in installation, operation, and maintenance, they may not be the best solutions for every single scenario. Most of those systems using jet ejectors suffer from a number of disadvantages, for example:                (a) they are usually horizontally submersed into the liquid adjacent to the bottom of a typically 4 to 6 meters deep reactor. They use high-velocity coherent jets which are adapted to overcome water pressure at the bottom of the reactor, consequently providing high liquid flux and relatively much more energy to entrain desired quantities of atmospheric air. Thus, they are often adapted to feed forced air provided by air blowers which also require additional energy. Despite the optimization efforts the energy utilization per unit volume is still considered high        (b) none of them can control entraining gas flow at single nozzle level, the control mechanisms are usually outside and centralized to provide uniform air flowrate to every nozzle. This arrangement is potentially a disadvantage for adjusting aeration levels in a plug-flow reactor.        (c) having submersed jet mixing apparatus approximately 5 meters under water makes the system vulnerable for any operation and maintenance concern such as potential nozzle clogging. In that case, the reactor is required to be emptied or alternatively a professional wastewater diver must be hired for the underwater repair work. There are some retrievable apparatuses also available, but retrieving a 5-meter-wide and 5-meter-tall nozzle manifold is not a simple maintenance job.        
The prior art comprises submersed liquid jet ejectors horizontal or with an approximate 45° trajectory angle (to horizontal XY-plane) and vertically plunging jet ejectors over and above the liquid surface. The following are examples of technical articles from plunging liquid jet literature and related prior arts or patents.    H. Chanson, R. Manasseh (2003) “Air entrainment processes in a circular plunging jet: Void Fraction and Acoustic Measurements”, Journal of Fluids Engineering, ASME, September 2003 Vol. 125 pg 910.    T. Bagatur and N. Sekerdag (2003) “Air-entraintment characteristics in a plunging water jet system using rectangular nozzles with rounded ends” ISSN 0378-4738, Water SA Vol. 29 No. 1 Jan. 2003.    Ito, K. Yamagiwa et al (2000) “Maximum Penetration depth of Air bubbles entrained by vertical liquid jet”, Journal of Chemical Engineering of Japan Vol 33 pg. 898    Liu, G., Evans, G. M., (1998). “Gas entrainment and gas holdup in a confined plunging liquid jet reactor”, Proceedings of the 26th Australian Chemical Eng. Conference, (Chemeca 98), Port Douglas, Australia.
The above technical articles focus on a plunging liquid jet over and outside the liquid body where the liquid jet is in contact with the gas above the liquid surface in a reactor so that it will usually entrain ambient gas by the impingement at the liquid surface (such as disclosed in PCT patent applications, WO/2005/108549 and WO/1992/03218).
Based on literature and model study results for the present invention, a jet ejector can generate a ratio of entrained air to motion water 2 to 4 air per water (volume/volume) as also disclosed in U.S. Pat. No. 4,690,764. However, dispersing and effectively dissolving of the entrained gas in an energy-efficient means still remains as a challenge for the prior art and this was mentioned above.
The plunging jet mix prior art suffer from a number of disadvantages:                (a) all of the above plunging jet aerators claim and rely on a jet ejector located over and outside of the liquid to be aerated, therefore air (gas) entraining is dependent on a high-speed coherent jet impingement on a liquid surface which can generate a high ratio of entrained air per liquid flux; however, the more the gas entraining, the less dissolution efficiency. Therefore, those prior art items often utilize relatively excessive hydraulic energy deliberately to shear gas bubbles into very small size to increase bubble penetration depth (energy efficiency suffers).        (b) despite being not very energy efficient, those prior art items have another potential problem: coalescence of small bubbles into larger bubbles due to high concentration of small bubbles and in turn causing dissolution efficiency and unwanted foaming problems.        (c) even though some of those prior art items disclose a controllable gas entraining mechanism, most of them are strictly designed for maximizing gas entrapping by coherent liquid jet impingement and therefore they are not capable of turning the gas completely off and accommodating anaerobic mixing conditions        (d) none of those prior art items disclose entraining of any other fluid other than an oxygen-containing gas or air, therefore they are not designed to entering any other fluid to accommodate alternating process conditions such as anaerobic, anoxic, aerobic, and oxic conditions in a liquid reactor        (e) none of those prior art items disclose any particular floatable matter de-stratification mechanism in addition to keeping settleable matter in suspension. Most of them do not define specific mixing patterns for energy-efficiency; their focus is strictly on entraining oxygen-containing gas or air since concomitantly provided mixing is usually chaotic and very high in both degree and energy utilization        
It is an object of the present invention to provide a liquid treatment process and apparatus which reduces at least one of the aforementioned disadvantages.